A thermoelectric conversion material and a thermoelectric conversion module using the same are used for applications such as, for example, cooling and power generation as devices converting heat into electric power. For example, when direct current is applied to the thermoelectric conversion material, heat transfer occurs from one surface to the other surface, which generates a heat absorbing surface and a heat generating surface. This phenomenon is called the Peltier effect, and when the thermoelectric conversion material is formed as a module and the heat absorbing surface is allowed to contact a target desired to be cooled, the target can be cooled without providing a movable part. On the other hand, when a temperature difference is provided to both ends of the thermoelectric material, an electromotive voltage proportional to the difference is generated. This phenomenon is called the Seebeck effect. For example, when one surface of the module is allowed to contact a target that is wasting excessive thermal energy and the other surface is cooled by air cooling or water cooling, thermal energy can be converted into electrical energy. That is, waste heat energy can be recovered as electrical energy. The thermoelectric conversion module using the Seebeck effect has received attention as a power generation device in recent years, and development is becoming active as a new utilization of the thermoelectric conversion module.
The most well-known material that generates the above thermoelectric conversion phenomenon effectively is a bismuth-tellurium based material. A module using the bismuth-tellurium based material has been already practically used for the application of cooling which utilizes the Peltier effect, and is also used for an application of adjusting temperature of a laser diode for optical communication. Accordingly, use of the bismuth-tellurium based material for the application of power generation is also considered. However, power generation efficiency using the thermoelectric conversion material (bismuth-tellurium based material) has temperature dependence, therefore, the material has not been widely used for the application of power generation.
The above point will be explained in detail. As a physical property value indicating properties of the thermoelectric conversion material, there is a Seebeck coefficient S (unit: V/K). This is a numeric value indicating a size of the electromotive voltage caused by the temperature difference, which is a numeric value indicating a voltage per a unit temperature difference. The Seebeck coefficient takes a positive or negative value according to the thermoelectric conversion material. This is determined according to whether carriers in the thermoelectric conversion material are holes or electrons, and it is common that, when the Seebeck coefficient is a positive value, the material is called P-type, and when the Seebeck coefficient is a negative value, the material is called N-type. As another physical property value indicating properties of the thermoelectric conversion material, there is an electric resistivity ρ (unit: Ω·m). When the electromotive voltage based on Seebeck effect is generated, electricity flows in the thermoelectric conversion material, and electric power which can be taken in the application of power generation is proportional to a product of the voltage and the electric current. Therefore, when the electric resistivity is low, electric power which can be taken is increased. That is, the above two physical property values directly affect power generation ability of the thermoelectric conversion material, which are represented by a numeric value as a power factor PF (unit: W/mk2) (hereinafter also referred to as merely “PF”) calculated by the following formula (1).
                    [                  Formula          ⁢                                          ⁢          1                ]                                                            PF        =                              S            2                    ρ                                    (        1        )            
Moreover, a thermal conductivity κ (unit: W/m·K) is also a value indicating properties of the thermoelectric conversion material though the value is not a physical property value directly acting on power generation. This indicates that, in the case where the Seebeck effect is desired to be generated with respect to fixed thermal energy, the temperature difference in the material is hardly generated when the thermal conductivity of the thermoelectric conversion material is too high. Therefore, the temperature difference can be increased in a material having a lower thermal conductivity, as a result, a power generation amount can be increased. As an index obtained by combining a Seebeck coefficient S with an electric resistivity ρ and a thermal conductivity κ, there is a dimensionless performance index ZT which is represented by the following formula (2).
                    [                  Formula          ⁢                                          ⁢          2                ]                                                            ZT        =                                            S              2                                      ρ              ·              κ                                ×          T                                    (        2        )            
The reason why the above dimensionless performance index ZT includes an absolute temperature T(K) is that respective numeric values have temperature dependence. However, as the power generation amount itself is represented by PF as described above, ZT is used as a criterion representing thermoelectric conversion performance. That is, there is a case where ZT indicates a large value when the thermal conductivity is extremely small, ZT may be a large value, however, the power generation amount is not increased unless PF is simultaneously increased. PF of the bismuth-tellurium based material becomes highest in the vicinity of normal temperature and tends to be reduced as the temperature is increased, therefore, the bismuth-tellurium based material is not suitable for being used at high temperature.
When large electric power is desired to be obtained by using the thermoelectric conversion material, it is necessary to take a large temperature difference. Recently, an attempt in which heat in the vicinity of 300° C. discharged from plants or motors of cars and so on is converted into electricity and effectively utilized is being made. However, PF is reduced as the temperature is increased in the bismuth-tellurium based material against an aim of increasing the power generation amount by increasing the temperature difference as described above. That is, it is difficult to increase the power generation amount due to the temperature dependence and consideration of a new material is inevitable.
In order to use a thermoelectric conversion member in an range exceeding 300° C., there is a problem that if is difficult to maintain not only the power generation performance but also quality of the thermoelectric conversion member. That is, the thermoelectric member itself easily reacts to oxygen in air as the temperature increased, and oxidation proceeds. Generation of a portion where oxidation occurs means that a crystal structure forming the thermoelectric conversion member is broken, and a function as the thermoelectric conversion member is deteriorated.
Patent Literature 1 identified below relates to a thermoelectric conversion module. In the literature, to suppress diffusion and oxidation of metal elements on a joint surface of the thermoelectric conversion element (thermoelectric conversion member) by arranging a diffusion prevention layer is described. FIG. 6 shows a schematic cross-sectional view of a thermoelectric conversion element of a thermoelectric conversion module and a joint surface thereof disclosed in Parent Literature 1. As shown in FIG. 6, a P-type thermoelectric conversion element 101 and an N-type thermoelectric conversion element 102 are joined to a high-temperature side electrode 108 covered with an electrode protective layer 107 through a solder layer 106 in the thermoelectric conversion module. Moreover, a first diffusion prevention layer 103 and a second diffusion prevention layer 104 for preventing metal diffusion are arranged between respective thermoelectric conversion elements 101/102 and the solder layer 106, and a solder joint layer 105 for joining to the solder layer 106 is further arranged therebetween. Then, Patent Literature 1 discloses that metal diffusion and oxidation in the joint between the thermoelectric conversion elements 101 and 102 can foe suppressed by adopting the structure.